I've only been on the air for three months so probably lack perspective but I have to ask:
Isn't "R" redundant?
If you didn't copy some or all of the other op's transmission, then you can ask for repeats of appropriate parts. Or if you are a newbie, send QRS PSE. If conditions warrant it, send SRI NO CPY, QSB, QRM, QRN, whatever.
Otherwise: just assume "R" without having to send it.
OTOH it does, in some respects, give you a short breathing time in a QSO to figure out what to say next. A bit like BT is used sometimes: not just as punctuation but as a kind of breathing space for thought.
Also I suppose the "R" is a kind of courtesy, a bit like saying "I hear you" and of course, analogous to "Roger" in phone traffic. Most CW ops are nothing if not courteous.
73 de Martin, G3EDM